Light-emitting elements utilizing electroluminescence have been currently under active research and development. In a basic structure of the light-emitting element utilizing electroluminescence, a layer containing a light-emitting substance (hereinafter, the layer is referred to as a “light-emitting layer”) is interposed between a pair of electrodes. By voltage application between the pair of electrodes of the light-emitting element, light can be emitted from the light-emitting substance.
Particularly among light-emitting elements utilizing electroluminescence (EL), the one in which an organic compound is used as the light-emitting substance can be formed by stacking thin films. Because the element can thus be reduced in thickness and weight and can have a larger area easily, it is expected to be used for a planar light source. Furthermore, the light-emitting element is expected to have emission efficiency exceeding that of an incandescent lamp or a fluorescent lamp, and thus has attracted attention as a light-emitting element suitable for lighting equipment.
The light-emitting element can emit light of a variety of colors depending on the kind of light-emitting substance. A light-emitting element which can emit white light or light of color close to white with high efficiency has been particularly required to be applied to lighting.
As a light-emitting element which can emit white light, for example, a white light-emitting element in which a plurality of EL layers having emission peaks in the red, green, and blue wavelength ranges are stacked has been proposed (e.g., Patent Document 1). In addition, a white light-emitting element in which two EL layers having emission peaks in the wavelength ranges of complementary colors (e.g., blue and yellow) are stacked has been proposed (e.g., Patent Document 2). Note that such a structure of stacked EL layers may be called a tandem structure.
A tandem light-emitting element includes, between an anode and a cathode, a plurality of EL layers with an intermediate layer interposed therebetween. Light emitted from the plurality of EL layers can be collectively extracted, and accordingly current efficiency can be made higher than in a light-emitting element including a single EL layer.
The intermediate layer has a structure in which a charge generation layer, an electron-relay layer, an electron-injection buffer layer, and the like are stacked. Note that these layers are not necessarily provided and may be selected as needed.
In manufacture of a tandem light-emitting element, an anode is formed over a substrate, and a hole-injection layer, a hole-transport layer, a light-emitting layer, and an electron-transport layer of a first EL layer are formed over the anode. An intermediate layer is next formed over the electron-transport layer of the first EL layer. When the intermediate layer includes an electron-injection buffer layer, an electron-relay layer, and a charge generation layer, the intermediate layer can be formed by successive evaporation of lithium oxide (Li2O), copper phthalocyanine (abbreviation: CuPc), and a substance having a high hole-transport property, for example. Next, a hole-transport layer, a light-emitting layer, an electron-transport layer, and an electron-injection layer of a second EL layer are formed over the intermediate layer, and a cathode is formed over the second EL layer. Thus, the tandem light-emitting element in which the two EL layers are stacked can be formed.
In the manufacturing process of a light-emitting element, an electrode serving as an anode of the light-emitting element is formed over a substrate before the other electrode serving as a cathode is formed as described above in some cases, and in other cases, an electrode serving as a cathode is formed over a substrate before the other electrode serving as an anode is formed. An element structure formed in the former cases of the manufacturing process is referred to as an “ordered structure”, and an element structure formed in the latter cases is referred to as an “inverted structure”. Not only these element structures of the light-emitting element are mutually inverted over a substrate, but also the element structures may differ from each other depending on the difference in manufacturing processes. In the tandem structure, the stacking order of the plurality of layers forming the intermediate layer is also inverted (e.g., Patent Document 3).
Furthermore, a structure in which an oxide semiconductor (OS) is used for a semiconductor layer of a field effect transistor (FET) for controlling a light-emitting element has also been proposed. In particular, since an OS-FET using indium gallium zinc oxide (IGZO) as an oxide semiconductor is an n-type transistor in which the majority carriers are electrons, a combination of the OS-FET and the inverted-structure light-emitting element whose cathode is connected to the OS-FET has been proposed as the way to improve element characteristics (e.g., Non-Patent Document 1).